The present disclosure relates to a wavelength tunable laser module and a method of controlling a wavelength of the wavelength tunable laser module.
With an increase in amount of information communication, optical signals are demanded to be multiplexed with narrower wavelength spacing in a wavelength division multiplexing (WDM) communication field in which a plurality of optical signals having different wavelengths are multiplexed and transmitted simultaneously in a single optical fiber. For multiplexing the optical signals with narrower wavelength spacing, it is necessary to highly accurately control a wavelength of laser light emitted from a laser element as a signal.
In recent years, what is called a flexible grid method is introduced into the frequency spacing of laser light to efficiently use an optical transmission wavelength bandwidth instead of a common fixed grid method in which the frequency spacing is fixed to 25 GHz or 50 GHz. The flexible gird method allows wavelengths to be arranged randomly with different frequency spacing.
It has been examined to utilize a semiconductor laser module using an etalon filter for communication employing the flexible grid method. The etalon filter has a wavelength transmission characteristic changeable by controlling a temperature thereof. The semiconductor laser module using the etalon filter splits a part of laser light emitted from a semiconductor laser element to the etalon filter and controls a temperature of the semiconductor laser element on the basis of power of the split light transmitted by the etalon filter, thereby controlling the wavelength of laser light emitted from the semiconductor laser element. The etalon filter has a periodic wavelength transmission characteristic. The periodic wavelength transmission characteristic is shifted in a wavelength direction depending on the temperature of the etalon filter. As a result, the semiconductor laser module using the etalon filter may control laser light emitted therefrom such that laser light has desirable wavelength and power. For example, refer to Japanese Patent Application Laid-open No. 2012-33895.
When the flexible grid method is achieved on the basis of the extension of the related technique, the flexible grid method has a limit. For example, when the whole of C-band is covered by channels based on a common frequency grid with 50 GHz spacing, the number of channels is approximately 100. When the related technique is simply extended to a frequency grid with spacing of 0.1 GHz, which is an example of the grid spacing of the wavelength tunable laser module for providing the flexible grid method, the number of channels is approximately 50,000, which is 500 times of that of the 50 GHz grid. It is, however, not practical, from mass production and cost viewpoints, that calibration is performed on all of the approximately 50,000 channels and laser driving conditions and control target values as the results of the calibration are stored in a memory.